gh-116417: Move 4 limited C API test files to _testlimitedcapi (#116571)

Move the following files from Modules/_testcapi/ to
Modules/_testlimitedcapi/:

* bytearray.c
* bytes.c
* pyos.c
* sys.c

Changes:

* Replace PyBytes_AS_STRING() with PyBytes_AsString().
* Replace PyBytes_GET_SIZE() with PyBytes_Size().
* Update related test_capi tests.
* Copy Modules/_testcapi/util.h to Modules/_testlimitedcapi/util.h.